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IOWA ACADExMY OF SCIENCES 



of the Mississippi valley, and even of other portions of 

 North America. 



Figure 4.— Shows the relative thickness of the members of the Mississippi valley coal measures 

 north and south. 



The thickness of the coal measures of the Arkansas 

 valley as estimated by Branner* is nearly 24,000 feet. If 

 present correlations be correct the highest of these beds in 

 Arkansas are not above the horizon of the Bethany 

 limestone of Kansas. For the deposition of such an enor- 

 mous sequence there must have existed exceptional con- 

 ditions. The great development of the coal measures in 

 Arkansas is not widespread, but is confined to a compara- 

 tively limited area. 



The noteworthy feature in the lithology of the Arkansas 

 coal measures is their make-up of shales and sandstones, 

 with an almost total absence of marked limestones. While 

 this characteristic is remarkable through such an extensive 

 succession, it points clearly to attendant physical condi- 

 tions that are unmistakable, and that are now known to 

 be in perfect harmony with the historical record of other 

 parts of the region. 



The Lower Carboniferous formations are well understood 

 in Arkansas. It is now known that the Boone cherts are 



Am. Jour. Sci., (4), Vol. II, p. 235, 



